Tracing the Origin of Metal Ions in Mississippi Valley-Type Ore Deposits: Constraints from Lead Isotope Studies of Black Shales from the Midcontinent United States

Mississippi Valley-type (MVT) lead-zinc ore deposits found in the midcontinent United States have isotopic signatures that indicate the potential involvement of a single highly radiogenic source mobilized by hydrothermal fluids or the mixing of two sources of Pb with differing radiogenic signatures. A genetic link between black shales and major MVT ores has been thought to exist because of (1) the presence of mature hydrocarbon and brine fluid inclusions within many MVT ores, (2) the association of the hydrothermal fluids that are linked to MVT ore formation with organic compounds and oil-like droplets, and (3) the highly radiogenic nature of the ores. The analysis of 69 shale samples from 21 stratigraphic units from across the midcontinent United States revealed variations in Pb concentrations and isotopic compositions across different Cambrian- to Pennsylvanian-age shales. Shales with high organic carbon content, particularly those of Pennsylvanian age, displayed elevated Pb abundances, suggesting their formation under anoxic conditions was conducive to the preservation of organic carbon binding with Pb ions. However, the Pb isotope signatures of most black shales are generally less radiogenic than the ores, suggesting that they were not a source of radiogenic Pb. Several samples of the Devonian-Mississippian Chattanooga Shale and the Ordovician Polk Creek Shale have significant quantities of radiogenic Pb that more closely resemble those of the MVT ores. This suggests that certain shales may have directly interacted with the hydrothermal fluids that precipitated the MVT ores. Some shales may have acted as an aquitard as the hydrothermal fluids were migrating through the subsurface, causing some shales to become more enriched in radiogenic Pb than others.